昆栏树科Nordenskioldia属在新疆阿勒泰古新统的发现及其意义

在我国新疆阿勒泰古新统发现的北方诺登斯基果(Nordenskioldiaborealis)主要具以下特征:果序轴有2个分枝,上面着生多个无柄的果实。着生果实的果序轴基部明显膨大,并由此向果序远轴端(下部)逐渐变细。果实近球形或卵圆形,由15个分果组成,分果围绕一中央轴排成一轮。将被前人鉴定为扁叶对枝柏(DitaxoicladusplaniphyllusGuoetSun)的种鳞化石重新鉴定为北方诺登斯基果。分析诺登斯基果属在我国的地质地理分布,并结合现生昆栏树科的地理分布,认为:1)诺登斯基果属在我国地质历史时期曾有着广泛的分布,但自古新世后其分布区很可能从北向南退缩;2)诺登斯基果属比昆栏树属(Trochodendra)分布的气候带更广,前者比后者具有更强的气候适应性,这印证了诺登斯基果属不是昆栏树属的直接祖先的推测。

DISCOVERY OF NORDENSKIOLDIA(TROCHODENDRACEAE) FROM THE PALEOCENE OF ALTAI, XINJIANG AND ITS IMPLICATIONS

Nordenskioldia borealis Heer, 1870, emend. Crane, Manchester et Dilcher, 1991, an extinct species of Trochodendraceae, is recognized and described from the Paleocene on the south of Ertix River of Altai city in Xinjiang. The key characteristics of these fruit specimens are that the infructescence axis has two braches with numerous sessile fruits. The axis is markedly expanded at the position where the two branches are given off. It rapidly tapers to the distal end. Fruits are subglobose or ovate in shape and schizocarpic at maturity. Each fruit is composed of 15 schizocarpic fruitlets arranged in a whorl around a central column. Specimens formerly described as seed scales of Ditaxoicladus planiphyllus Guo et Sun in Altai flora are reexamined and assigned to Nordenskioldia borealis. Based on the analysis of the geological distribution of Nordenskioldia in China, it is found that this genus occurred in four sites of China through geological time, namely, the Altai as mentioned in the present article, the Late Cretaceous Taipinglinchang Formation and Paleocene Wuyun Formation in Jiayin, Heilongjiang, and the Late Paleocene Changchang Formation in Qiongshan, Hainan. In consideration of the distribution of living species of Trochodendraceae, it is suggested that Nordenskioldia once widely distributed in China, and that it probably shrank back to the low latitude areas after Paleocene. In addition, it is concluded that Nordenskioldia has the advantage of Trochodendron in respect of adaptation to climate, and this confirms the opinion that Nordenskioldia is not the direct ancestral to Trochodendron although Nordenskioldia is currently the stratigraphically older known taxon.